The present invention relates to multilamp photoflash units and, more particularly, to improved means for providing electrical insulation between adjacent elements in compact multilamp photoflash units.
Numerous multilamp arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226, 3,912,442, 3,935,442, 3,937,946, 3,941,992, 3,952,320 and 4,017,728 and referred to as a flip flash) employs high-voltage type lamps adapted to be ignited sequentially by successively applied high voltage pulses from a source such as a camera-shutter actuated piezoelectric element. The flip flash unit comprises an elongated planar array of eight high-voltage type flashlamps, each having a pair of lead-in wires connected to a printed circuit board by means of eyelets secured thereon. The circuit board is provided with switching circuitry for causing sequential flashing of the lamps, and an array of respectively associated reflectors are positioned between the lamps and the circuit board. The reflectors for the lamps can be made as a single reflector member shaped to provide multiple individual reflectors for the lamps. The reflector member preferably is electrically conductive, such as being made of metal or metalcoated plastic, and is electrically connected to an electrical "ground" portion of the circuitry on the circuit board. Thus, the reflector member functions as an electrical shield reducing the possibility of accidental flashing of lamps by an electrostatic voltage charge on a person or object touching the unit. Such accidental flashing is particularly prone to occur in this instance as the lamps are high voltage types requiring a firing voltage of 500 to 4,000 volts, for example, at low current.
Although an electrically conductive reflector is desirable, as has just been explained, care must be taken to prevent the reflector unit from shorting the circuitry on the circuit board, which is located immediately behind the reflector unit, thus preventing the circuit from flashing the lamps. According to the above-referenced U.S. Pat. No. 3,894,226 Hanson, this problem is solved by interposing between the reflector unit and circuit board a sheet of electrically insulating material to prevent shorting of the circuitry on the circuit board by the conductive reflector. The insulating sheet may be of a transparent plastic a few thousandths of an inch thick, which transmits sufficient radiation from flashing lamps to actuate adjacent radiation switches in the switching circuitry, and also to actuate flash indicators located behind the circuit board.
With respect to the switching circuitry, it has been found advantageous to use radiant-energy-activated disconnect switches in series with the lamps. Such switches permit more rapid sequencing of the lamps within the array by electrically removing them from the circuit after flashing. The use of disconnect switches also permits the lamp design to be optimized (e.g., with respect manufacturing cost) without regard for whether or not the lamp may internally short circuit.
A number of radiant-energy-activated disconnect switches have been described in the prior art. Examples are U.S. Pat. Nos. 3,532,931 Cote et al, 3,726,631 De Graaf et al, 3,728,067 De Graaf et al, 3,692,995 Wagner, 3,666,394 Bok. A particularly fast acting disconnect switch is described in the aforementioned U.S. Pat. No. 4,017,728 of Audesse et al, wherein the switch element comprises a piece of electrically conductive, heat shrinkable, polymeric material which is positioned so as to be in operative relationship with the radiant output from the series-connected lamp during the ignition thereof. It has been found, however, that physical contact between the midportion of the disconnect switch and other objects, such as either the circuit board on one side or the transparent insulating sheet on the other side, promotes conductive heat loss and nonpositive switch functioning. Accordingly, each switch element is attached at both ends to the lamp-igniting circuit with the midportion of the element being spatially suspended to avoid contact with heat-absorbing surfaces. In a specific embodiment, the circuit board is provided with a plurality of apertures respectively in alignment, via corresponding reflector apertures, with the flashlamps mounted thereon, and each length of conductive heat shrinkable material comprising a switch is attached to the circuit board so as to bridge a respective aperture therein. In this manner, the midportion of the material is spatially suspended to avoid contact with the circuit board. In addition, the transparent insulating sheet has a plurality of holes located in alignment with respective reflector apertures over the midportions of the disconnect strips, thereby avoiding contact with the insulating sheet. Upon ignition of its associated lamp, the midportion of the switch element rapidly shrinks and separates in response to the radiant output of the lamp, thereby providing a quick-acting, reliable open circuit to high voltages.
It has been found, however, that high voltage sparking sometimes occurs from the conductive disconnect switch to the conductive reflector through the respective hole in the insulating sheet. Since the disconnect switches comprise part of the "hot" circuit and the reflectors are connected to the "common" circuit, such spark-over constitutes an electrical short circuit and promotes lamp failure. It also sometimes happens that a disconnect switch curls during functioning so as to give rise to a web of the conductive switch materials that extends up through the hole in the insulating sheet. The presence of such switch residue projections gives considerably reduced gap lengths and higher likelihood of reliability-decreasing spark-over.